The preparation of poly(phenylene sulfide) can be accomplished by heating p-dichlorobenzene in the presence of a partially dehydrated mixture of sodium sulfide and N-methylpyrrolidone as disclosed in U.S. Pat. No. 3,354,129. U.S. Pat. No. 3,354,129 is incorporated by reference into this disclosure. After completion of the polymerization process, the poly(phenylene sulfide) can be recovered by flash evaporation of the volatile components, i.e., N-methylpyrrolidone, water, unreacted p-dichlorobenzene and volatile by-products of the polymerization reaction. U.S. Pat. Nos. 4,056,515 and 4,060,520 teach such a process and are incorporated by reference into this disclosure. A significant volatile by-product of the polymerization reaction is phenol. Other by-products include diphenyl ether and N-methyl-succinimide.
In order to recover N-methylpyrrolidone the flashed volatiles are fractionated in two successive distillation processes. The flashed volatiles are first passed to a "lights column" where water, unreacted p-dichlorobenzene and the more volatile by-products are removed overhead. The less volatile by-products (including phenol, diphenyl ether and N-methyl-succinimide) and N-methylpyrrolidone accumulate at the bottom of the "lights column." These kettle bottoms are subsequently fed into an "N-methylpyrrolidone recovery column" which generally operates at a temperature higher than that of the first column.
The overhead of the second recovery column is relatively pure N-methylpyrrolidone. Phenol, an impurity, accumulates at the bottom of the column so long as the phenol concentration in the kettle bottoms remains low (generally less than about 20 weight percent) and the temperature of the kettle bottoms remains low (generally below about 460.degree. F.). Above these levels the phenol codistills with N-methylpyrrolidone. Because N-methylpyrrolidone is recycled to the polymerization step and because phenol adversely affects polymerization, codistillation of the phenol is highly undesirable.
It is well known that N-methylpyrrolidone (also called N-methyl-.gamma.-butyrolactam) and phenol form a 1:1 adduct as disclosed in the Journal of Organic Chemistry 29, 3122-3124 (1964) incorporated by reference herein. This adduct formation apparently explains why phenol accumulates in the kettle bottoms even though it has a lower boiling point than N-methylpyrrolidone. Under the distillation conditions of the "N-methylpyrrolidone recovery column" a considerable excess of N-methylpyrrolidone relative to the amount of phenol present is necessary to preserve the thermal stability of the adduct. For this reason, a significant amount of N-methylpyrrolidone (about four times the amount of phenol) cannot be recovered by distillation since it must remain in the kettle bottom to bind phenol and thus prevent the phenol from distilling overhead and contaminating the N-methylpyrrolidone. This incomplete recovery is costly in that a valuable reagent is lost and kettle bottom disposal problems are magnified.
The kettle bottoms of the N-methylpyrrolidone recovery distillation column can be treated with neutral or acidic water to extract N-methylpyrrolidone. Although the water favors N-methylpyrrolidone over phenol (and the other impurities) a limited amount of phenol does enter the aqueous phase. The problem associated with the above-described water extraction of N-methylpyrrolidone in this concomitant transfer of some phenol to the aqueous phase. Removal of the transferred phenol is a necessary prerequisite to the recycling of N-methylpyrrolidone to the polymerization reactor because of the previously described interferring nature of phenol.